When pressure meets light: detecting the photoacoustic effect at the origin

Abstract

Modern optical microscopy1, including confocal microscopy, two-photon microscopy and optical coherence tomography (OCT), has revolutionized life sciences by providing detailed information of biological samples with cellular and subcellular resolutions, and has become an essential tool for biomedical research labs. However, optical microscopy typically has a limited penetration depth of ~1 mm in biological tissue due to strong optical scattering2, 3. Moreover, with respective contrast mechanisms, confocal and two-photon microscopy usually rely on fluorescent labeling of the samples and OCT still lacks sensitivity to many biological functions. In contrast, optical-resolution photoacoustic microscopy (OR-PAM) has emerged over the last decade as a complementary imaging tool to the existing optical microscopy by taking advantage of its unique optical absorption contrast4. By acoustically detecting the optical absorption in the tissue, OR-PAM has been proven a powerful tool for anatomical, functional and molecular imaging with endogenous or exogenous contrast agents. In particular, using hemoglobin as the endogenous optical absorber, OR-PAM currently represents the most sensitive blood detector and has been widely used for in vivo imaging of the blood perfusion and oxygenation, especially for cancer and brain studies. Nevertheless, the acoustic detection in OR-PAM is a double-edged sword; on the one hand, it provides a relatively deep penetration with one-way optical attenuation and negligible acoustic attenuation, but on the other hand, the acoustic detection typically needs a coupling medium, such as water and ultrasound gel, between the tissue surface and the ultrasound transducer. The need for acoustic coupling has become one of the major factors that has hindered the wide adoption of OR-PAM by biomedical researchers whenever the biological samples are not compatible with an aquatic environment. Therefore, contact-free detection of the photoacoustic signals (that is, without the need for acoustic coupling) has captured the attention of the photoacoustic imaging community and resulted in many exciting advancements. If successful, the contact-free photoacoustic technologies will free up the working space and greatly expand the territory of PAM applications.